The processes involved in "glycolysis" have been described in detail in numerous reference works on biochemistry, such as the standard textbooks entitled Biochemistry by A. Lehninger or L. Stryer (any edition), and in Medical Physiology by A. Guyton.
Briefly, in all mammalian cells, molecules of glucose (a six-carbon sugar) are continuously being broken apart in a series of enzymatic steps. This results in the formation of a three-carbon intermediate called pyruvate. Pyruvate is the dissociated (ionized) form of pyruvic acid; it predominates at physiological pH's. The reactions which create pyruvate release a relatively small amount of energy.
In cells which have sufficient oxygen supplies, a substantially larger amount of energy is generated in a second set of enzymatic reactions, in which pyruvate is metabolized all the way to carbon dioxide and water. This process is often referred to as "oxidative respiration" or "aerobic respiration". It requires and consumes oxygen as part of the process.
In cells which do not have sufficient oxygen (such as in the brain of a person suffering a stroke, in the heart muscle of someone suffering a heart attack, in someone suffering cardiac arrest, suffocation, or drowning, and to a lesser extent in the muscles of someone engaged in strenuous exercise), any reserve oxygen in affected cells is used up within a few minutes, and the cells quickly become subject to conditions of inadequate oxygen supply, at varying degrees of severity. In such "hypoxic" tissue, when there is not enough oxygen to support fully oxidative respiration, the series of enzymatic steps that consume pyruvate is diverted into a different pathway, and pyruvate is converted into lactic acid instead of carbon dioxide.
The enzymatic pathway that converts pyruvate into lactic acid is usually called anaerobic glycolysis. This series of anaerobic reactions yields substantially less energy than oxidative respiration; however, it does yield some energy, and it removes metabolites that would impede and slow the initial set of reactions that generate pyruvate from glucose. Therefore, anaerobic glycolysis is used by cells as a routine mechanism when muscles are exercised, and as a backup mechanism for providing some level of energy in crisis situations such as stroke, heart attack, cardiac arrest, or asphyxiation.
Lactic acid, a three-carbon acid, readily dissociates at physiological pH ranges, to form the lactate ion, which is negatively charged. Since lactate and lactic acid co-exist (in equilibrium concentrations) in the blood, they are referred to interchangeably herein; lactate accumulation means the same thing as lactic acid accumulation.
In healthy animals (including humans), lactate is readily converted back into glucose or pyruvate in the liver and in certain muscles (including heart muscle tissue). This prevents lactate buildup, and the levels of lactate that are normally present in blood do not cause any damage. However, in patients and animals which are severely stressed or suffering from certain diseases (such as various types of terminal cancer), or in situations involving local ischemia (i.e., inadequate blood flow, as occurs during a stroke or heart attack), abnormally high accumulations of lactate and lactic acid can damage tissue and cells, by increasing the acidity of blood or cellular fluids to levels that inhibit the functioning of various essential enzymes. In addition, lactate specifically binds to and inhibits the enzyme phosphofructokinase; this can shut down the backup process of anaerobic metabolism, thereby making an ischemic or hypoxic crisis even worse.
Therefore, an abnormal increase in lactate concentration generally can be regarded as bad for cells and tissue. It leads to a sensation of fatigue in affected muscles during exercise, and during a genuine crisis (such as a stroke or heart attack), it leads to an array of adverse effects that are generally referred to by physicians under the terms "acidosis" or "lactic acidosis". In either situation, the buildup of lactic acid in blood or tissue can be regarded as a wasteful and inefficient use of energy supplies, which becomes especially detrimental under conditions of scarcity.
In addition to the problem of wasting scarce energy supplies, lactate can also act as a toxic poison if it accumulates to severe levels. It can act directly as a neurotoxin, and it can also inhibit or poison enzymes that are crucial to glycolysis, such as phosphofructokinase. If poisoned by severe lactate buildup, phosphofructokinase cannot recover and carry out its essential glycolytic functions, even after oxygen supply is restored. This poisoning of an essential glycolytic enzyme can severely aggravate the creation of oxygen-containing "free radicals," which aggressively attack cell membranes and other biomolecules, after blood supply is restored to oxygen-starved tissue.
Accordingly, methods which can efficiently reduce the creation or accumulation of lactate in blood or tissue are useful in treating patients suffering from ischemia (inadequate blood supply) and hypoxia (inadequate oxygen supply), and in various other conditions, such as certain types of diabetes and epileptic seizures, and certain types of surgery involving cardiopulmonary bypass (i.e., so-called "heart-lung" machines).
There have been prior proposals to treat such patients with dichloroacetate (DCA), which can stimulate the oxidative removal of lactate by increasing the activity of an enzyme called pyruvate dehydrogenase (PDH). For review articles discussing the enzymatic, pharmacological, and metabolic effects of DCA, see Crabb et al 1981 and Stacpoole 1989.
In the past, proposals for using DCA to reduce lactate levels in human patients via intravenous injection or infusion have taught that DCA should be injected at least three times per day, or more frequently, since it is rapidly cleared from circulating blood in the human body. For example, in reports involving administration to humans, Curry et al 1985 used five infusions per day; Irisigler et al 1979 reported three infusions within 4 hours; Stacpoole et al 1988 used four infusions over 24 hours; and Wells 1980 reported that DCA clearance in rats is also fairly rapid (e.g., Evans 1982 and U.S. Pat. No. 4,631,294 (Barsan 1986)).
It should be noted that dogs (which are widely used in cardiac and circulatory studies) demonstrate DCA metabolic clearance rates that are markedly slower than humans or rats. For example, Wells et al 1980 reported that the half-life of DCA in humans was 31 minutes, and that DCA had been completely eliminated within 5 hours. By contrast, the half-life of DCA in dogs is reported to be 19-24 hours, which is about 40 times longer than the 0.5 hour half-life of DCA in humans (Lukas et al 1980, and Ribes et al 1979). Accordingly, dogs cannot be used as reliable models or predictors for the pharmacokinetic effects of DCA in humans.
Evans 1982 showed that in rats, DCA concentration in the liver declined rapidly with time, and that the activation of the pyruvate dehydrogenase enzyme complex by DCA also dropped off rapidly with time. These results, from tests on rats, were directly contrary to the results gathered by the Applicants in human tests, as described herein. Evans et al 1981 administered DCA orally, in 100 mg/kg doses, either once, or daily for seven days. The peak DCA concentrations observed were about 25 .mu.g of DCA per gram of liver tissue; if liver tissue was in equilibrium with plasma (which is suggested by the rapid clearance of the drug after the cessation of DCA administration), then this is only about 25 .mu.g/mL, which was only about 1/10 of the plasma concentration seen in human patients dosed intravenously, in the studies described below.
It also should be noted that administration of DCA to reduce ischemic or hypoxic damage has never been approved by the U.S. Food and Drug Administration for human use, except in experimental clinical trials. Accordingly, administration of DCA is not an option that is currently available to patients suffering from stroke, cardiac arrest, etc., or to physicians treating such patients, even though proposals for using DCA to treat these conditions were first published and patented more than a decade ago.
This invention discloses a new method whereby DCA may be administered less frequently than three times daily, to a patient suffering from adverse lactate buildup. This new dosage regimen is supported by two new discoveries: (1) the beneficial effects of DCA in reducing lactate concentrations persist for longer than eight hours in humans, thereby eliminating the need to administer DCA every eight hours; and (2) the beneficial effect of DCA in reducing lactate concentrations in humans persists, leading to a sustained plateau effect, even after DCA concentration has spiked and has dropped to greatly reduced or undetectable levels in the blood.
In other words, it has been shown that the beneficial effects of DCA (i.e., reducing serum lactate levels) persist beyond the period of time that the drug persists in the circulation. These discoveries can substantially facilitate the medical use of DCA in patients suffering from elevated serum and/or tissue lactate concentrations due to ischemia, hypoxia, and various other conditions.
Accordingly, one object of this invention is to disclose an improved method of administering DCA to patients in need of such therapy, in a practical and cost-efficient manner.